Electrostatic chucks (ESC) are presently being developed and used for the fabrication of semiconductor wafers. The concept of an ESC is to replace mechanical wafer clamping mechanisms with an electrostatic clamping mechanism in order to realize advantages such as reduced particles, better temperature control, and reduced edge exclusion zones.
One of the properties of ESCs (monopolar and bipolar) is that the clamping force increases as the voltage applied to the ESC increases. A clamping force which holds the wafer on a temperature controlled pedestal of the electrostatic chuck is needed to enhance thermal conductance (usually aided by a backside gas pressure on the wafer) between the wafer and the pedestal. This provides better wafer temperature control and uniformity.
U.S. Pat. Ser. No. 5,103,367 (Horwitz et al.), issued on Apr. 7, 1992, discloses an electrostatic chuck (ESC) for semiconductor wafers using A.C. field excitation. The chuck comprises first and second aligned electrodes embedded in a thin dielectric film that define a substantially planar surface. The first and second electrodes are each excited by a low-frequency AC supply to produce sine wave field of controlled amplitude and phase that provides a low resultant voltage on the wafer surface. A third electrode disposed parallel to the first and second electrodes acts as a shield electrode or a reference point for the first and second electrodes. By controlled rates of voltage application and removal, low voltage gradients are obtained on the wafer and no retentive forces exist in the dielectric medium. In one embodiment, a low AC amplitude excitation of the chuck enables capacitive current sensing of the position of the wafer relative to the dielectric film, thereby enabling simple control of the voltage application to the first and second electrodes.
U.S. Pat. No. 5,325,261 (Horwitz), issued on Jun. 28, 1994, discloses an electrostatic chuck (ESC) system for holding a body (such as a semiconductor wafer) with improved release thereof. The ESC system comprises a holding device such as an electrostatic chuck having a surface for contacting the body, an electrode and applying means for applying a drive voltage to the electrode in order to electrostatically grip the body to the surface, and determining means for determining a value of a drive release voltage to be applied to the electrode in order to enable a release of the body. The determining means preferably comprises monitoring means for monitoring the motion of the body as the drive voltage is varied. The applying means is capable of applying a voltage signal over the drive voltage in order to cause oscillating motion of the body. This oscillating motion of the body is monitored by position sensing circuitry to produce a demodulated sense output in accordance with the oscillation. As the drive voltage is varied, a point will be reached where the body is released. This point can be determined by monitoring the demodulated AC sense output.
Although a minimum amount of threshold voltage is usually required to clamp a wafer to a pedestal of an electrostatic chuck, a voltage which is excessively high will "press" the wafer onto the pedestal and cause abrasion between the wafer and the pedestal surfaces. This is undesirable and can lead to particle problems and/or a shortened chuck life. Thus a trade-off exists between applying too much voltage to the chuck, which can shorten the ESC life and cause particle problems, and not applying enough voltage such that the wafer is not sufficiently clamped to the pedestal of the electrostatic chuck.
It was found that wafers with a large inherent bow or warpage (usually a result of film stress) need a higher ESC chucking or clamping voltage in order to provide a sufficient clamping force. In other words, an optimum ESC clamping voltage depends on the extent of wafer warpage or bow, and, therefore, the higher the inherent wafer warpage the greater the ESC clamping voltage that is needed. Presently, a single ESC clamping voltage is used for one or more groups of wafers which is much higher than is required. Factors such as a heater set point of the electrostatic chuck and estimates of inherent wafer warpage are often taken into consideration to determine the value of the ESC clamping voltage for the one or more groups of wafers. There are still instances in which wafers do not properly clamp due to an excessive amount of inherent wafer bowing or warpage. There are also wafers which can be clamped with a lower voltage than the single ESC clamping voltage used for a group of wafers such that backside abrasion and scratching can be reduced or avoided.
Since the inherent warpage varies from wafer to wafer, it is desired to provide a method and apparatus that will measure the inherent bow or warpage in a wafer, and use such measured data to apply a substantially optimum chucking or clamping voltage for each wafer to an electrostatic chuck in order to substantially avoid excessive amounts of backside abrasion and scratching of the wafer while the wafer is being processed.